1. Field of Invention
The present invention relates to liquid crystal light valve image projection systems. More specifically, the present invention relates to applying a spatially varying electric field across a liquid crystal cell to produce a transparent image having a spatially varying optical density.
2. Summary of Prior Art
Liquid crystal cells are useful in display, optical processing, printing, photolithographic, and related image projection applications. In these applications they serve as light valves or light modulators which control the transmission or reflection of light from a light source to a receiver or receiving surface. When suitable spatial electronic addressing means is incorporated in a liquid crystal light valve system, spatially varying patterns can be written on the liquid crystal cell. The liquid crystal cell then becomes a spatial light modulator. When suitable illumination optics, typically including a light source and optical condenser, and suitable projecting optics, typically including projection lens with appropriate aperture, are included in a liquid crystal light valve system, the liquid crystal cell then becomes an electronic slide which determines and controls a projected image.
The spatial variations of the image on the liquid crystal cell correspond to differences in the molecular orientation of the liquid crystal in different regions of the liquid crystal layer. The spatially varying differences in molecular orientation are converted into spatially varying light intensity variations by means of suitable polarizing optics or suitable apertures to selectively pass and block scattered or refracted light from these spatially different regions. Various possibilities have been reviewed in detail in the literature (see Kahn, "The Molecular Physics of Liquid Crystal Devices", Physics Today, May 1982.)
In many liquid crystal devices, the entire image area of the liquid crystal cell is switched, typically by electrical or thermal means, into a texture with uniform order (or disorder), thereby providing a uniform image background. One or more spatially varying images with different textures are then superposed, by a spatial addressing means, onto the uniform image background in order to create an image on the liquid crystal cell, and hence on a receiving surface, with the desired spatial variations in intensity and contrast.
The means for creating the spatial variations of texture in the liquid crystal fall into two general categories; that in which the variations are the result of phase changes in the material (see Kahn), and those in which changes in optical retardation of polarized light occurs due to changes in the orientation of the molecular axis of the liquid crystal. These changes typically are caused by local application of electric field (see Boller et al., "A Low Electro-optic Threshold in New Liquid Crystals", J. Applied Physics, Vol. 43, May 1971).
Devices which use optical retardation effects to modulate light are typically non-storage and need to be refreshed at or near the flicker-frequency threshold of the human eye, about 17ms/frame. While these devices have frame writing times on the order of several milliseconds, they are not capable of image storage. Consequently, high information content images require high bandwidth addressing and large capacity digital memories. Prior art devices utilized liquid crystal devices (typically with nematic liquid crystals) in which a frame of information in optical form is impressed on the photoconductor which then transiently modulates the liquid crystal and must be refreshed at approximately 60 times/sec.
Additionally, in these photo-conductive addressed devices, the addressing light and the interrogation light are applied at the same time. Therefore, a light blocking layer is typically disposed between the photoconductor and the liquid crystal layer to avoid activating the photoconductor by the action of the interrogation light.
Devices which use phase change effects to modulate light are typically storage-type and only need to be addressed once. A smectic or cholesteric liquid crystal has two stable states, or mesophases. In one state the liquid crystal is transparent and in the other it is scattering. Those regions on the microscopic scale where the liquid crystal is in the scattering state are called scattering centers. Thermal methods are primarily used to induce phase changes. In general, the liquid crystal material is heated above the nematic-isotropic transition temperature and then allowed to cool. If slowly cooled, a transparent state results, if quickly cooled, a scattering state results. Local thermal phase changes are, for example, induced by laser scanning, and are relatively slow, with frame writing times in the order of several minutes for high resolution images. However, the image is stored in the liquid crystal until erased.
Most prior art optical projection systems which utilize a spatial variation in mesophase to modulate light create scattering images on clear backgrounds. When these scattering images are projected from the liquid crystal cell with suitable optics, they generally create images which are dark on a bright background. It is often desirable to create a bright image on a dark background. The optical system used to transform a scattering image on a clear background to a bright image on a dark background sacrifices brightness to achieve this objective.
An additional drawback to prior art systems which use scattering cells is that the ability to create greyscale, a continuous tone from black to white, is unavailable. Most prior art scattering systems are binary. That is, each point is either entirely in a scattering state or a clear state, and thus appears either full dark or full bright when projected.